1. Field of the Invention
This invention relates to an optical recording element for recording and retrieving information by irradiating, for example, laser beams, and a method of manufacturing the same.
2. Description of the Prior Art
The known high density optical recording system comprises radiating laser beams whose diameter is narrowed to less than 1 micron on a thin recording layer to produce pits therein or change the structure of said thin recording layer for the recording of information; irradiating laser beams on the thin layer in which information is recorded and retrieving information according to the percentage reflection of laser beams from said thin layer or the percentage permeation of laser beams through said thin layer.
FIGS. 1A and 1B show the conventional optical recording element used with the above-mentioned optical recording system. FIG. 1A is a front view of said optical recording element, and FIG. 1B is a cross sectional view thereof. A disk-shaped optical recording element (hereinafter referred to as "a disk") 1 is provided at the center with a hole 2 for a spindle by which the disk is rotated. The disk 1 is formed by mounting a thin recording layer 5 on a glass substrate 4 by means of, for example, thermal deposition. A tremendous number of pits 3 are formed in a vertical pattern at a smaller pitch than 2 microns. Where information is recorded by forming pits 3 on the surface of the recording layer 5, then said layer 5 should preferably be prepared from a metal or noncrystalline material which melts at a temperature of 400.degree. to 500.degree. C. and evaporates at a lower temperature than 2,000.degree. C. Where information is recorded by the structural change of the recording layer 5, then said recording layer 5 should preferably be formed of a vitric semiconductor material. A protective layer 7 is fixedly mounted on the recording layer 5 by means of an adhesive 6 to prevent dust from being deposited on the recording layer 5 and saving said layer 5 from damage during handling.
However, the conventional optical recording element constructed as described above is accompanied with such various problems or drawbacks as are described below. The first problem is that at the time of recording, heat energy escapes. Where laser beams are irradiated on the recording layer 5 through the protective layer 7 to record information in the optical recording element 1, then heat generated in the recording layer 5 partly escapes to the glass substrate 4. Therefore, heat energy required for recording is lost by the amount which has thus escaped. Power has to be applied in a sufficient amount to make up for lost heat energy in order to fully carry out the formation of pits on the surface of the recording layer 5 or the structural change thereof. This requirement unavoidably leads to an increase in the size and cost of an optical recording system.
The second problem is that where information is recorded on the surface of the recording layer 5, then difficulties arise from the evaporation of the atoms of the material constituting said recording layer 5, which adversely affects the formation of pits 3, making it difficult to produce pits 3 in the desired distinct form.
The third problem is that the adhesive 6 exerts a harmful effect. That is, said adhesive 6 chemically reacts with the material of the recording layer 5 or protective layer 7, or bubbles and dust particles included in the adhesive 6 produce an objectionable effect on the recording and retrieval of data.
The fourth problem is that the disk is produced at a low efficiency. The recent trend goes toward the enlargement of the capacity of the optical recording element 1. Therefore, said element 1 is desired to be increased as much as possible in capacity. At present, a glass substrate having a diameter of 300 to 400 mm is generally used. It seems advisable to form optical recording disks having such a large diameter as 400 mm by placing two or three glass disks separately in a large evacuation apparatus and thermally deposit a recording layer on said glass disks after the evacuation is completed. However, this process which can produce only a few disks in 3 to 4 hours is far from being adapted for the quantity production of said disks. A memory device generally requires 1,000 to 2,000 optical recording disks. Therefore, the disk is demanded to have such a structure and be prepared from such a material as can reliably allow for tonnage production of an entirely different level from that which has been attained by the conventional technique. Further requirements are that the disk should be commercially available at a low cost, and a tremendous number of pits having a smaller diameter than 1 micron have to be formed all over the surface of a disk having a diameter of 400 mm.
A lens used to narrow the diameter of, for example, laser beams to 1 micron has to be operated over an extremely short range of distance as less than 1.5 mm. Further, rigid limitations are imposed on the flatness precision of the surface of the glass substrate 4, because the focal point of the above-mentioned lens has to be always set on the recording layer 5. With the conventional optical recording element 1, therefore, the surface of said glass substrate 4 should be polished with as high precision as possible.